1. Field of the Invention
The present invention relates to an optical information reproducing apparatus for reading information which has been recorded on an optical storage medium such as an optical recording medium or a magneto-optical disk by irradiating the optical storage medium with light, and a method of manufacturing the same.
2. Description of the Related Art
In recent years, optical storage mediums are widely used. Such optical storage mediums include, for example, an optical recording medium such as a CD-ROM (compact disk-read only memory) for reproduction only, a write-once type optical dick, and a rewritable optical disk. In an Optical information Reproducing apparatus utilizing such an optical storage medium, various types of optical pickup are used for reading information from said optical storage medium. There has been developed a technique by which the size, the weight and the price of the optical pickup are reduced. In the developed technique, by using diffraction devices having diffraction gratings formed by holography technique, the number of components of an optical system in the optical pickup is reduced.
An exemplary optical pickup having diffraction devices is shown in FIG. 10. Below the recording face of an optical recording medium 90 as the optical storage medium, a semiconductor laser device 100 is disposed. Light emitted from the semiconductor laser device 100 is incident on the optical recording medium 90 through a diffraction device 120, a diffraction device 130, a collimator lens 140, and en objective lens 150. At one side of the semiconductor laser device 100, a light receiving device 160 which outputs an electric signal corresponding to the intensity of the received light is disposed. In the optical recording medium 90, information is recorded in pits along a track 183.
A lens actuator (not shown) is connected to the objective lens 150. By the lens actuator, the objective lens 150 is moved in a focusing direction and a tracking direction. The focusing direction used herein is a direction in which the objective lens 150 is moved toward end away from the optical recording medium 90. The tracking direction used herein is a direction perpendicular to a direction along which the track 183 extends at a position irradiated by the light. By this movement, the light from the objective lens 150 is converged on the pit of the optical recording medium 90. In a case where the optical recording medium 90 is in a disk shape, the tracking direction is the radial direction of the optical recording medium 90.
The diffraction device 120 has a diffraction grating for diffracting the light emitted from the semiconductor laser device 100, whereby the emitted light is split into three beams, i.e., a main beam and two sub beams. The diffraction device 130 has two diffraction gratings 131 and 132 which are disposed in parallel along an X--X' direction parallel to the direction along which the track 183 of the optical recording medium 90 extends. The diffraction gratings 131 and 132 are respectively parallel to this X--X' direction. Between the diffraction gratings 131 and 132, a boundary 133 is formed which extends in a Y--Y' direction perpendicular to the direction along which the track 183 of the optical recording medium 90 extends. The light receiving device 160 is divided into five portions 161-165. The portions 161 and 162 are divided by a dividing line 166.
In such an optical pickup with the abovementioned construction, the laser light introduced into the diffraction device 120 from the semiconductor laser device 100 is diffracted with the diffraction device 120. As a result, the laser light is split into three beams, i.e., a zero-order diffracted beam as the main beam, plus/minus first-order diffracted beams as the sub beams. The thus obtained main beam and cub beams are further diffracted with the diffraction gratings 131 and 132 of the diffraction device 130. A zero-order diffracted beam as a main beam which is obtained by the diffraction in the respective diffraction gratings 131 and 132 is converged on the optical recording medium 90 through the collimator lens 140 and the objective lens 150.
After the light converged on the optical recording medium 90 is reflected from the optical recording medium 90, the reflected light is introduced into the diffraction device 130 through the objective lens 150 and the collimator lens 140. The reflected light is diffracted with the diffraction device 130. Diffracted light from the respective diffraction gratings 131 and 132 is incident onto the light receiving device 160. At this time, the first-order diffracted beam of the main beam obtained by the diffraction of the diffraction grating 131 is converged at the dividing line 166 and forms a light spot R.sub.1. The main beam which is obtained by the diffraction of the diffraction grating 132 is converged on the portion 163 and forms a light spot R.sub.2. The sub beams which are obtained by the diffraction of the respective diffraction gratings 131 and 132 are converged at four points, i.e., two points on the portion 164 and two points on the portion 165, and form light spots R.sub.3 -R.sub.6.
Now, a focusing error of the converged spot on the optical recording medium 90 is explained. In a case where the light from the semiconductor laser device 100 is properly converged at an information pit on the optical recording medium 90, as is shown in FIG. 11B, the light spots R.sub.1 -R.sub.6 each in a perfect circular shape are formed in the respective portions 161-165 and on the dividing line 166 of the light receiving device 160. In a case where the distance between the optical recording medium 90 and the objective lens 150 is smaller than the focal length, as is shown in FIG. 11A, the light spots R.sub.1 -R.sub.6 each in an expanded semicircular shape are formed on the respective portions 161, 163, 164, and 165 of the light receiving device 160. In a case where the distance between the optical recording medium 90 end the objective lens 150 is larger than the focal length, as is shown in FIG. 11C, the light spots R.sub.1 -R.sub.6 each in an expanded and inverted semicircular shape are formed in the respective portions 162, 163, 164, and 165 of the light receiving device 160.
A tracking error at the converged spot on the optical recording medium 90 is described with reference to FIGS. 10 and 12. The two sub beams are converged at positions which ere symmetrical with respect to the main beam as a point of symmetry and which ere displaced slightly in the Y--Y' direction and displaced somewhat more in the X--X' direction from the converged spot of the main beam. In a case where the main beam is properly converged on the track 183 in which the information pits are arranged on the optical recording medium 90, as is shown in FIG. 12B, the two cub beams at converged spots 181 and 182 are respectively incident on the track 183 in the same area. Therefore, the reflective intensities of the reflected beams from the optical recording medium 90 by the converged spots 181 and 182 are equal to each other. On the contrary, in a case where the converged spot 180 of the main beam 90 displaced with respect to the information pit of the optical recording medium 90 in the tracking direction, as is shown in FIG. 12A or 12C, the two sub beams at the converged spots 181 and 182 are respectively incident on the track 183 in different areas. Therefore, the reflective intensities of the reflected beams of the two sub beams from the optical recording medium 90 are different from each other.
From the portions 161-165 of the light receiving device 160 on which the light spots R.sub.1 -R.sub.6 are formed, signals 81-85 corresponding to the intensities of the received beams are output, respectively. Based on the signals S.sub.1 -S.sub.5, calculations ere performed between the signals by an electronic circuit element (not shown). Specifically, based on a difference S.sub.1 -S.sub.2 between the signals S.sub.1 and S.sub.5, focusing error signal FES is calculated. Based on a difference S.sub.4 -S.sub.5 between the signals S.sub.4 and S.sub.5, a tracking error signal TES is calculated. Based on the sum S.sub.1 +S.sub.2 +S.sub.3 of the signals S.sub.1, S.sub.2, and S.sub.3, an information signal RF is calculated.
In order to ensure that the recording end the reproduction of information are accurately performed, based on the focusing error signal FEB end the tracking error signal TES, the objective lens 150 is moved and the position thereof is adjusted by the lens actuator in both the focusing direction and the tracking direction. By this positioning, the emitted light from the semiconductor laser device 100 is accuretely converged on the information pits of the optical recording medium
In the optical pickup having the construction shown in FIG. 10, as is shown in FIG. 13, the semiconductor laser device 100 and the light receiving device 160 are mounted at predetermined positions in a can package 185. In this situation, the can package 185 is airtightly sealed. To the upper surface of the can package 185, a transparent glass plate 186 is adhered. On the lower face of the glass plate 186, the first diffraction device 120 is formed by etching. On the upper face of the glass plate 186, the second diffraction device 130 is formed by etching. On the bottom face toward the inside of the can package 185, as is shown in FIG. 14, a stem 187 is provided. At one side of the stem 187, the semiconductor laser device 100 is mounted. On the upper face of the stem 187, the light receiving device 160 is mounted. On the bottom face of the can package 185, a photo detector 188 for monitoring optical power which detects the power level of the laser light from the semiconductor laser device 100 is provided adjacently to the stem 187. Based on the output from the photo detector 188, the semiconductor laser device 100 is controlled so as to provide a constant output.
In the above-mentioned conventional optical pickup, the semiconductor laser device 100 and the light receiving device 160 are separately and independently fabricated, and then the semiconductor laser device 100 and the light receiving device 160 are fixed on the different faces of the stem 187. Accordingly, it is difficult to match the positions Of the semiconductor laser device 100 and the light receiving device 160 with high accuracy, and the relative positions thereof are likely to be shifted. Specifically, it is very difficult to suppress the error in the height direction to several tens of micrometers or smaller. In a control signal such as the .focusing error signal FES end the tracking error signal TES, an offset component caused by the errors in the height direction of both the devices can be eliminated by positioning the diffraction device 130. However, if the offset component is too large, some improper conditions occur such as that the waveform of the control signal is distorted by the positioning. Since the .semiconductor laser device 100 and the light receiving device 160 are provided on the separate faces of the stem 187, the can package 185 cannot be made thinner, This prevents the optical pickup from being reduced in size,
The electronic circuit element for calculating the output signal from the photo detector 188 is usually provided on a wiring board which is different from the wiring board on which the optical pickup is provided. In this case, for example, the signal frequency of a reproduced information signal from a compact disk is in the vicinity of 1 MHz, and the signal frequency of a reproduced information signal from a laser disk is in the vicinity of 2 MHz. That is, the reproduced information signal is a high-frequency signal, which means that the reproduced information signal may easily include noise. Therefore, it is necessary to take sufficient care in designing wirings.
In the conventional optical pickup, the semiconductor laser device 100 and the light receiving device 160 are separately and independently fabricated, and then attached onto the Same stem 187. Therefore, a registration error of about several tens of micrometers occurs between the semiconductor laser device 100 and the light receiving device 160. Accordingly, it is necessary to compensate an offset component in a control signal caused by the registration error, by adjusting the position of the diffraction device 130. In the process for adjusting the position of the diffraction device 130, it is necessary to actually reproduce the information from the optical recording medium 90. Moreover, the offset amounts vary depending on the employed optical pickups. Therefore, the assembling process in manufacturing the optical pickup is complicated, which prevents the optical pickup from being produced at lower cost and from having a higher quality.
Moreover, in the conventional optical pickup, since the semiconductor laser device 100 is mounted on the side face of the square stem 187, the can package 185 is large in size, which prevents the optical pickup from being reduced in size.